The invention relates to a water-repellent and oil-repellent finishing agent for a sweater and a preparation method thereof, and belongs to the technical field of textile finishing agents. According to the water-repellent and oil-repellent finishing agent and the preparation method thereof, amino acid is generated by hydrolyzing silk fibroin, abundant reaction bonding points are utilized, beta-cyclodextrin is introduced by bonding, a modified silicone oil component with the hydrophobic and oleophobic functions is loaded on the surfaces of fabric fibers and acts on the surfaces of wool fabrics,so that the surfaces of the fibers are covered with a large amount of non-hydrophilic groups, and therefore the water-repellent and oil-repellent performance of the wool fabric can be improved; and octamethylcyclotetrasiloxane, aminoethylaminopropyl polydimethylsiloxane, a silane coupling agent HD-109 and a silane coupling agent HD-110 serve as raw materials, modified organic silicon oil is synthesized under the action of a catalyst and through a mode of polymerization with functional groups contained in side chains of casein, due to the polarity of surface groups of a polymer, the surface groups can interact with hydroxyl groups, carboxyl groups and the like on the surfaces of the fibers, so that siloxane main chains are directionally attached to the surfaces of the fibers, a directionalgrating is formed, and therefore the water-repellent and oil-repellent properties are remarkably improved.